1. Technical Field
This disclosure relates generally to signal comparison circuitry and in particular, but not exclusively, to image sensor readout circuitry.
2. Background Art
Comparator circuits are often found in semiconductor devices such as CMOS image sensors. A popular form of comparator circuit is an asynchronous comparator circuit, which receives an input signal and provides an output signal representing a comparison of the input signal to some reference signal. In an asynchronous comparator circuit, the input signal directly triggers generation of the output signal. Thus, output signals of asynchronous comparator circuits are typically prone to signal noise. For example, such output signals tend to exhibit threshold jumping which is interpreted by other digital logic as a rapid switching between binary logical states.
Another type of comparator circuit is a synchronous comparator circuit, which is operated with a clock or other strobe signal to enable a comparison decision based upon the input signal. Triggering of an output signal in a synchronous comparator circuit further depends upon such a strobe signal. However, output signals of synchronous comparator circuits are typically prone to oversaturation when a strobe signal triggers generation of the output signal. As a result, such output signals tend to exhibit a sudden and extended switching from a logical state which correctly represents a comparison to the opposite, incorrect logical state.
As integrated circuit fabrication techniques continue to improve, successive generations of smaller and/or faster semiconductor devices are increasingly sensitive to the performance limitations in such comparator circuits.